The molecular structure of plant starch depends on the degree of polymerization and branching of the component polyglucan chains. Starch granules consist mainly of two different kinds of polymer structures: amylose which primarily consists of unbranched chains of about 1000 glucose molecules, and amylopectin which is much larger than amylose and branches every 20-25 glucose residues. Some starch granules also contain phytoglycogen, a highly branched starch.
A principal enzyme that determines the extent to which these different starch forms are present in a particular starch granule is starch synthase which is involved in elongating the polyglucan chains of starch, transferring the glucose residue from ADP-glucose to the hydroxyl group in the 4-position of the terminal glucose molecule in the polymer. Starch synthases from different plant sources have different catalytic properties (e.g., rate of chain elongation, affinity for different substrates), in part accounting for the differing fine structure of starch granules observed from plant to plant, and even from one developmental stage to another for a given plant.
Expectedly, starch synthase has been the focus of a number of studies. Starch synthase is localized in the plastid, where starch formation in plants occurs. Starch synthase activity has been observed bound to the starch granule (“granule-bound form”) or in the supernatant of crude extracts (“soluble form”). The number of isoforms and their expression patterns vary with the plant species and the developmental stage. For example, in maize endosperm, there are at least four starch synthase isoforms, two soluble and at least two granule-bound. In potato tuber, three soluble starch synthase isoforms and at least two granule-bound isoforms have been identified. One of the three soluble isoforms in potato tuber, SSI, is expressed more in leaves than in tubers.
The Waxy locus encodes a granule-bound starch synthase responsible for amylose synthesis and has been cloned from several plant species. Genes encoding different isoforms of soluble starch synthases have been isolated as well. Certain starch synthases remain uncharacterized in detail and it is believed that additional isoforms have yet to be discovered. The chemical properties of a particular starch is ultimately determined by its structure, so that manipulation of starch structure at the molecular level, by modulating the activity of enzymes like starch synthase involved in starch biosynthesis provides a tool for designing starch to suit a particular need, or for obtaining starch of uniform composition. For example, sorghum waxy mutants contain amylopectin exclusively, and their glutinous grains produce wine with higher quality and specific fragrance compared with those of wild-type. Accordingly, genes encoding novel isoforms of starch synthase may prove useful in producing starch structures with novel chemical properties. Disclosed herein are nucleic acid fragments encoding starch synthase including starch synthase isoform V isolated from leaf and young seedling tissue which if expressed in storage organs like seeds or tubers may lead to altered reserve starch composition. Sequence for a starch synthase isoform V from Vigna unguiculata (NCBI General Identification No. 4582783) as well as a closely related maize sequence for a partial starch synthase protein (PCT WO 97/26362; U.S. Pat. No. 6,211,436) have been disclosed previously.